Tech FAQ

Setting timing:

One of the most common questions I get: how to set timing to a figure that’s not within the timing marks on my engine.  The best answer is to upgrade to a dial-back timing light.  The way they work is by entering your timing setting (such as 14 BTDC) into the timing light via a knob or buttons, then proceed to line up the TDC marks on the pulley.  The timing light automatically offsets the light to take advantage of your existing TDC marks.  Some engines like the TR4 only have marks for 10 BTDC, so if you want to set your timing to 14, you use the figure of 4 degrees at the light for 10+4=4.  Simple timing light, simple math.  Most auto parts stores will loan you a premium quality light for free if you don’t want to buy one.  

Spark plug wires:

Testing plug wires is as simple as using an Ohm meter. Google the process if you’re unfamiliar or buy a copy of Rick Astley’s electrical book based on the MGB, but it carries over to all British cars. The fundamentals are good with any car. Plug wires operate their best with a resistance value below 5000 Ohms, preferably under 1000 Ohms per foot of wire. Resistance in the carbon brush inside your distributor cap should read zero Ohms. Zero Ohms in the plug wires is good, but having some resistance actually drives up the coil voltage, resulting in a hotter spark at the plug. More is not always better. 1000 Ohms per wire is the sweet spot that gives strong voltage while not stressing your tune-up parts into premature failure.

Ignition coils:

All points type distributors need 3 Ohms of resistance in the coil’s Low Tension (LT) circuit (the 12V circuit as opposed to the HT plug wire circuit at 12000+ Volts.) Think of LT/HT as low volt/high volt. You can run a 3 Ohm coil fed with battery voltage or you can run a 1.5 Ohm coil with a 1.5 Ohm “ballast resistor” (meaning an external resistor) to equal 3 Ohms. They are additive. Some ballast resistors are ceramic and mounted on or near the coil, others are ballast wires hidden in the harness. Every 1.5 Ohms results in approximately a 2V drop at the coil, which you can measure with a volt meter if you are unsure if you have a hidden ballast wire. In the end, your points will be protected and will be reliable if you have 3 Ohms resistance in the LT circuit. Anywhere up to 4 Ohms is ok. You’ll find many 3 Ohm coils actually measure at 3.4, and that numbers rises with temperature. Its ok. Just get in the 3-4 Ohm ballpark and you’ll be good.

Advanced Timing:

Getting your car to run its best will mean experimenting with timing. Don’t believe what you read on the internet regarding timing. 32 degrees is not a magic number. No engine has a magic number. What worked in the 60’s no longer works, as fuel has changed WAY too significantly, even if you use premium non-oxy. It all has additives for EFI high pressure pumps and anti-foaming. Experimentation will result in finding the sweet spot where your engine runs its best. I like to shift timing settings in 3 degree increments and go for a drive. Your butt dyno should be able to tell you if there’s a change of 3-5 hp, if you know your car well. Find the point of most power, without any audible signs of detonation or shift in engine temps (typically up). If you make big changes by adding more timing, you may also need to add more fuel to balance out the fuel mixture. A sign of lean fuel mixture would be rising engine temps and/or popping through the exhaust while maintaining cruise speed with a steady throttle position. Numerically low timing settings result in sluggish off-idle performance as well as warmer than normal coolant temps. High timing settings results in poor drivability displayed in many different ways but usually a general loss of power over the entire rpm band.

Using an Air/Fuel meter:

Adding an oxygen sensor and gauge to your car has become inexpensive and is a great option for tuning your mixture precisely. I recommend a temporary install such as Innovate Motorsport’s tailpipe clamp for $80 in addition to their LM-2 system or many other portable options on the market. All of these systems have some level of skew, so the readings are guidelines – different for every car but there are some generalities. Typically, all testing should be done in a safe place, at WOT (wide open throttle). A good place to be for a non-boosted car is 12-13:1 AFR (air fuel ratio) during WOT use only. Some engine prefer the low side, others the high side, with variations for compression, carbs of choice, air velocity, port efficiency, etc… The trick is to experiment enough to find where your car runs its best and document those settings. This is dyno technology for the street, at a lot less than $100 an hour investment. The tailpipe clamp means you can tune all your cars, friends’ cars, club member cars, etc… Generally speaking, idle will land around 13:1 on a “wet manifold” car, meaning a carb will spray fuel into the air present in your manifold, unlike EFI where most systems leave the manifold dry and fuel is sprayed directly into the cylinder head. Your modern car may show a constant 15:1 AFR, due to the fuel injection placement, much more efficient at utilizing fuel than a carb. I’ve found that your idle mixture is correct when it doesn’t have a stable display – it will bounce from 12.5 – 13.5 in a way that allows you to see each exhaust pulse and some reversion between the pulses where fresh air pulls back into the tailpipe between exhaust pulses. A 3/8″ OD extension tube can be slipped into the tailpipe clamp to get readings further up the pipe, somewhat stabilizing the reversion readings at idle.

Installing a distributor in a disturbed (rebuilt) engine:

This may require an extra set of hands or a compression gauge. Turn the engine over from the crank bolt, with either a finger over the #1 spark plug hole, or with a compression gauge in the hole, until you see or feel the compression build in the cylinder. Since its a 4-stroke motor, the exhaust stroke will come up to TDC and build no compression, while the firing stroke comes up to TDC and DOES build compression. When you know you are on the compression stroke, turn the engine CW until you get to your timing setting, such as 15 degrees BTDC, and stop there. Do not back up to get there, turn the engine 2 more full revolutions to get back to that spot, always turning CW as you look at the front of the engine.

At this point, put a spark plug in the #1 plug wire, or use a clamp-style spark tester in the wire, and properly ground it to the engine. Make sure the distributor is properly installed with the rotor pointing roughly to the #1 spark plug wire terminal. Correct as needed. Loosen the distributor clamp enough so you can turn the distributor, and grab some welding gloves. Turn on the ignition key. Turn the distributor housing the opposite direction the shaft rotates, wearing the gloves or you’ll get shocked. You should see a spark at the plug. Turn it back past the starting point, and quickly repeat. You should be able to pinpoint the moment/position when spark happens at the plug. Turn off the ignition, and lock down the distributor clamp. Reinstall your #1 spark plug. If you have fuel present, the engine should start and run at the proper timing setting. Recheck with a timing light to confirm its set as you intended. This will work for points or most electronic ignitions, with the exception of some HEI-type systems that require a faster shaft rpm to develop a signal. This works beautifully on virtually all collector cars built before 1975.

Points or Pertronix?

This is one of the most frequently asked questions I get. My answer is almost always points, for performance and reliability, but it depends. If you have a distributor that is severely worn and the dwell angle refuses to stay stable, an electronic ignition will stabilize your dwell, providing better drivability. EVERY TIME this happens, every time you hear a story of a car running much better from installing electronic ignition, its because of this single factor. It was installed into a distributor that needs to be rebuilt – not cleaned and reassembled. Rebuilt. Most people don’t understand the difference because they don’t know what clearances are required or what advance curve is required to make their car run the way it used to run on leaded fuel of the decade the car was produced. In fact, most people don’t understand much about what the distributor does, and that’s ok. Just know its a critical function like valve timing or compression values, and if its wrong the car can run downright mediocre or worse. If you’re reading this, I know you want better than mediocre.

There are also a few cars that aren’t worth the effort to remove the intake assembly to get to the distributor, or cases where losing 10% power just doesn’t matter, although remember that when you give up this power, you are forcing your engine into a poor state of tune that can have other ramifications, such as fuel in your oil. The Lotus twincam comes to mind as one of those applications where access downright sucks. Then again, its a VERY expensive engine to rebuild, so the extra labor is worth it in many cases.

Here’s the biggest reason electronic ignitions don’t belong inside a distributor. Low voltage electronics do not work consistently when they are installed next to a high voltage source, unless you have plenty of room to install shielding, and even with that possibility, its rarely 100% effective. Inside your distributor spins a rotor, which throws an arc to each cap terminal, up to 50 times per second in a street car, 200 arcs per second in a 4 cylinder, 300 in a 6, 400 in an 8. This 15,000 Volt arc jumps within a couple inches of the 12V module that’s looking for a tiny magnetic or optical signal. Many times a second, an extra signal can be generated or one can be skipped. Even if the “misfire” rate is only 1 in 500 (usually its higher than that), you can be down 3, 5, 10 hp. I’ve seen a TR6 lose 39 hp on the dyno from installing an electronic ignition with no other changes. You’ll see this happen more on a chassis dyno than in an engine dyno cell since the alternator or generator, car wiring, and many other factors can turn the engine bay into a Faraday cage amplifying the problem – as if the distributor cap itself doesn’t already make a good Faraday cage. Other types of misfires, high resistance plug wires, improper or mediocre grounding, 5kOhm spark plugs (resistance type), and countless other common “issues” in collector cars can amplify issues when using ANY electronic ignition. Which one works perfectly? Points. They are a simple on/off grounding switch that rarely have issues. Yes, points need to be adjusted, but the adjustments are easy and only take a few minutes a year if you follow the simple guide below.

Adjusting points: breaking myths:

This used to be a simple monthly maintenance item, but has become a fear invoking, menacing, time consuming issue because people have just forgotten how to do it well. Here are a few simple tips to not only make it easy, but reduce how frequently you need to make adjustments.

If your manual says the point gap should be .014 – .016″, That DOES NOT mean set them to .015″. Never, ever. Points wear out over time. Set them to .016″, let them wear down to .014″, and repeat. In fact, my recommendation is that for many reasons including wear, play, slop, poor tolerances, you can set the point gap to .017″ or even .018″ and go from 3000-7000 mile intervals between adjustments. There will almost always be a break-in interval of 500-1000 miles, requiring an adjustment, similar to how you need to readjust valves or retorque a head gasket 500 miles after an engine rebuild. The key is to file and reinstall the same points, not new ones, for as many years as you can. You will get significantly longer adjustment intervals from a broken-in set of points than continually replacing them with new ones. Its not unusual for me to see a 50 year old set of points in an MGTD that outperform new replacements and appear to be good for another 50 years with a peak rpm of 7000+ engine rpm before bouncing becomes an issue.

Filing points can be done with a $2 point file from Harbor Freight or anywhere that has automotive tools. Don’t use sandpaper on points as it can leave behind a layer of silicon and resistance is not good – from residue or corrosion. An arc will eventually burn through it, but avoidance is easy. Occasionally I use a cut-off wheel to resurface badly pitted points, then file to clean.